The present invention relates to an electric motor, and more particularly to an electric motor casing.
Most motors have a field magnet on the stator, the field magnet comprising a plurality of field poles each having an iron core with a coil wound thereon and a yoke connecting the field poles to form a magnetic circuit. The yoke is made of a material selected primarily with a view to achieving required magnetic characteristics. Examples of yoke materials are cast iron, cast steel, silicon steel, and the like. Since these materials are iron-base metals and the yoke is required to have a certain volume, the yoke is heavy and normally responsible for most of the entire weight of the motor. Inasmuch as the yoke is highly sturdy in construction, it is generally employed as a motor casing or as a major portion of the motor casing.
Where a motor having such a yoke and casing is used on a ship or in other marine use, provision should be made for rendering the casing resistant to corrosion since the yoke materials as described above are highly susceptible to corrosion. It has been cutomary to coat the surface of the yoke and casing with a plated layer, a paint layer, a chemically deposited layer, or the like, or to make the yoke and casing of stainless steel that is highly corrosion-resistant.
The corrosion resistance of the yoke and casing with the surface layer deposited thereon can however be lost easily by physical contact particularly with a hard object having a sharp edge. The yoke and casing of stainless steel is disadvantageous in that since the stainless steel is not sufficiently ferromagnetic, the volume of the yoke should be so large as to ensure desired yoke performance, resulting in an increase in the weight and cost of the motor.